The invention relates to a vessel comprising at least two anchor lines connected respectively at a substantially fixed position near the bow and near the stem of the vessel.
From U.S. Pat. No. 3,583,354 a bow-stem spread catenary mooring system is known in which the vessel is maintained in a fixed position by means of four anchor lines, two of which are connected to the bow of the vessel and two at the stem. By applying the correct tension to these anchor lines it is possible in normal weather to maintain the ship in a reasonably fixed position. According to U.S. Pat. No. 3,583,354, during storn conditions the stem anchor lines are slackened, such that the vessel can weathervane around the bow through 90xc2x0.
From U.S. Pat. No. 3,822,663, a weathervaning vessel is known, which is anchored to the seabed by means of four anchor lines. The vessel is provided with a track which encircles the hull of the vessel. A plurality of movable carriages are guided along the track, one anchor line being connected to each carriage. The carriages can be placed at specific points along the track, each carriage being equipped with a winch for accumulating or discharging a respective anchor line. By moving the position of the carriages along the track, and by selectively paying in or paying out the anchor lines, the vessel can weathervane about a substantially fixed position point located between the bow and the stern. The known anchoring construction however is relatively complex and the anchor line arrangement covers a relatively large part of the seabed. Hence the anchor lines may interfere with the operating area for the risers and leaves little room for the anchoring spread of a drilling/workover rig when such a rig is anchored next to, or in close proximity to the drilling vessel.
It is therefore an object of the present invention to provide for a vessel which can weathervane within a predetermined area, in a controlled manner, using a relatively simple anchoring system. It is a further object of the present invention to provide a vessel comprising such a weathervaning anchoring system having a reduced anchoring layout and leaving a relatively large seabed area for the risers and for the anchoring spread of adjacent vessels such as drilling/workover rigs or shuttle tankers.
Thereto the vessel according to the present invention is characterised in that at least one anchor line comprises a first line section extending to a connection point located below keel level of the vessel and at least two branching anchor lines, each connected with one end to the anchor line in the connection point and connected with the other end to the sea-bed. With the above anchoring arrangement the vessel according to the present invention can be easily accessed along its sides. Thereby other vessels can easily approach and moor aside the vessel according to the present invention, and sufficient space is available for location of for instance flexible risers.
By providing at least one anchor line with two branching anchor lines, a very stable multiple point mooring system (at least a three point system) is provided which will leave enough play for the vessel to weathervane through angles less than 90xc2x0 in both directions. Preferably both the bow and the stem anchor lines each comprise at least two branching anchor lines.
In one embodiment, the anchor lines extend substantially vertically downward. Hereby access of the vessel is further increased.
As used herein, the term xe2x80x9csubstantially verticalxe2x80x9d shall mean a position from the first anchor line section being inclined from the vertical position by no more than 45 degrees.
In another embodiment, a buoyancy member is connected near the branching anchor lines. In this way, the stiffness of the anchor arrangement can be adjusted for varying the weathervaning action.
The term xe2x80x9canchor linexe2x80x9d as used herein comprises anchor cables, ropes, chains or combinations thereof.
In one embodiment, at least one anchor line extends through a generally vertical shaft extending through the hull of the vessel from deck level to keel level. In this way the anchor lines are located below the vessel, and the bow and stem are easily accessible as well.
Preferably a counterweight is connected to at least one of the anchor lines, such that the tension in the vertically extending part can be adjusted and the stiffness of the anchor line system can be varied for adjustment of the weathervaning action.
Another embodiment of a vessel according to the present invention comprises at least two anchor lines connected respectively at a substantially fixed position near the bow and near the stern of the vessel, at least one of the anchor lines being connected to a substantially fixed-position pulling device for lengthening or shortening of the anchor line, and is characterised in that the pulling device can vary the length of the at least one anchor line such that both anchor lines are maintained in a tensioned state, and the vessel can weathervane around the bow anchor line or the stem anchor line.
The invention is based on the insight that during normal operating conditions, a vessel will be subjected to wind and wave directions which are more or less constant for larger time periods. During these substantially constant conditions, the vessel only needs to weathervane through a relatively small angle such as between 0xc2x0 and 90xc2x0. For these conditions, by properly tensioning and slackening at least one of the anchor lines at the bow and stem of the vessel, an efficient and controlled weathervaning action can be achieved, without the need of a complex system of displaceable winches.
In one embodiment the pulling device, such as for instance a winch system, according to the invention varies the length of the anchor lines simultaneously. Variation of the anchor lines can take place under the control of a mean position sensor which actuates the winch system on the basis of the mean position of the vessel, the wind or wave directions, etc. The change in one of the above environmental characteristics will change the control signal and hence the stiffness of the anchor line system.
Preferably each anchor line is connected to the pulling device via a sheave located generally at the center line of the vessel. By the placement of the sheaves on the centre line of the vessel, a symmetric weathervaning action to both sides is achieved.
According to an embodiment of the vessel according to the present invention the anchor lines are connected to a single pulling device or winch. By operation of this single pulling device, bow and stem anchor lines can be simultaneously slackened or tightened to obtain a proper weathervaning position of the vessel. In one embodiment, a single anchor line passes from the bow, via the pulling device to the stern of the vessel, such that the length of the anchor line that is for instance taken in at the bow corresponds with the length of anchor line payed out at the stem.
By passing the anchor lines through a vertical shaft or hawser pipe extending from deck level to keel level, the sides of the vessel are easily accessible, and a shuttle tanker can be moored alongside the vessel according to the present invention without interference with the anchor lines.
By connecting at least one of the anchor lines below sea level to a counterweight, which in turn is connected to the seabed, the vertical arm of the weathervaning system can be adjusted and thereby the stiffness of the weathervaning system can be varied. Preferably the vessel according to the invention is connected to a subsea hydrocarbon structure via at least one riser. The riser is connected to the side of the vessel near midship where dynamic movements are minimum thereby increasing the longevity of these risers. The riser may pass through the vessel via the moonpool. As the movements of the vessel are limited to rotations of approximately 90 degrees, the risers also do not require complicated swivels at their attachment points as the riser itself can twist this angle.